Multi-frequency ground monitor current sensing

ABSTRACT

A signal generation module injects a signal in a pilot conductor with a DC component and one or more AC components. Pilot conductor current is injected in a grounding conductor of power cables connecting power source to load. A current monitor module monitors current in the pilot conductor or ground return, a DC detection module determines DC current present in the monitored current, an AC detection module determines AC current corresponding to each frequency of the AC components in the monitored current, a DC minimum threshold module determines if the DC current is below a DC current minimum threshold, an AC threshold module determines if any of the AC currents is below an AC threshold, and a trip module opens a contact after determining that the DC current is below the DC current minimum threshold or that at least one of the AC currents is below an AC threshold.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/871,448 entitled “MULTI-FREQUENCY GROUND MONITORCURRENT SENSING” and filed on Aug. 29, 2013 for Dale Curtis, which isincorporated herein by reference.

FIELD

This invention relates to monitoring ground current and moreparticularly relates to using a multi-frequency generator whenmonitoring ground current.

BACKGROUND

Mining is a very special environment that is by its very naturehazardous. Mine shafts are very limited physically, often include wetconditions, and can have explosive gases and dust. The mining industryhas a long history of accidents and fatalities. As a result,governmental regulations as well as company policies are geared towardmaking mining safer for those that enter and work in mines. Onegovernmental agency that regulates mining practice in the United Statesis the Mine Safety and Health Administration (“MSHA”). MSHA providesregulations as well as enforcement of the regulations.

Mining equipment is typically large and requires a significant amount ofpower. Mining equipment is also typically portable. To provide power tothe mining equipment, portable power sources are provided in and aroundmines. Due to the high power requirements of mining equipment as well asmines having long shafts, often mining power source have voltages thatare higher than are typically found in industrial situations. It is notuncommon for the mining power sources to provide power with voltagesbeing about 1 kilo volt (“KV”). In addition, due to the portability ofthe mining equipment and power sources, often power is provided usingflexible cables run without conduit. Due to the high voltages, exposedcables, wet conditions, etc., special consideration must be made forsafety of the electrical power systems in mining.

One way to increase safety and reliability of mining power systems is todetermine if grounding conductors are in place and properly connected.When grounding conductors are not properly connected, have shortcircuits, etc., when a fault condition occurs, electrical current canflow through the earth surrounding mine shafts as well as through miningequipment. Current flowing in unintended routes create a shock hazardfor miners both for fault conditions before overcurrent protectionreacts as well as steady state conditions when continuous current flowin unintended paths.

MSHA has provided regulations for monitoring current in groundingconductors. MSHA regulations regarding ground current were changedseveral years ago. Much of the electrical power system equipment in usetoday in mines today does not meet current MSHA requirements found intesting standard 30 of the Code of Federal Regulations (“C.F.R.”)section 75 and 30 C.F.R. section 77 with respect to ground currentmonitoring.

SUMMARY

An apparatus for monitoring ground current is disclosed. A system andmethod also perform the functions of the apparatus. The apparatusincludes a signal generation module that injects a signal in a pilotconductor. The signal includes a direct current (“DC”) component and oneor more alternating current (“AC”) components. Each AC component of theone or more AC components includes a frequency different from other ACcomponents of the one or more AC components. Current in the pilotconductor is injected in a grounding conductor of a set of power cablesconnecting a power source to a load. The grounding conductor isconnected to a ground return.

The apparatus, in some embodiments, includes a current monitor modulethat monitors current in at least one of the pilot conductor and theground return, a DC detection module that determines a DC currentpresent in the current monitored by the current monitor module, and anAC detection module that determines an AC current corresponding to eachfrequency of the one or more AC components present in the currentmonitored by the current monitor module. The apparatus, in someembodiments, includes a DC minimum threshold module that determines ifthe DC current is below a DC current minimum threshold, an AC thresholdmodule that determines if one or more of the AC currents determined bythe AC detection module is below an AC threshold, and a trip module thatopens a contact in response to one or more of the DC threshold moduledetermining that the DC current is below the DC current minimumthreshold longer than a DC current minimum time threshold and the ACthreshold module determining that at least one of the AC currents isbelow an AC threshold for longer than an AC current time threshold. Thecontact disconnects the power source from the set of power cables.

In one embodiment, the apparatus includes a DC maximum threshold modulethat determines if the DC current is above a DC current maximumthreshold and the trip module also opens the contact in response to theDC maximum threshold module determining that the DC current is above theDC maximum current threshold. In another embodiment, the DC currentmaximum threshold corresponds to a current that is above a DC current inthe pilot conductor or ground return that is at a level indicative of anoperating condition without a stray DC current component. In anotherembodiment, the apparatus includes a pilot wire diode (“PWD”) thatincludes one or more diodes, where the PWD is connected to the pilotconductor and to the grounding conductor and where the PWD has a voltagedrop when the signal is applied to the pilot conductor. In a furtherembodiment, the DC current maximum threshold corresponds to a currentthat is above a DC current in the pilot conductor or ground return whenthe load comprises a properly functioning PWD. In another embodiment,the PWD is located at the load.

In one embodiment, the trip module opens the contact in response to theAC threshold module determining that for at least two of the ACcurrents, an AC current is below an AC threshold for longer than an ACcurrent time threshold. In a further embodiment, the AC current timethreshold includes a first AC current time threshold and the one or moreAC components include three AC components and the trip module furtheropens the contact in response to the AC threshold module determiningthat each of the three AC currents is below an AC threshold for longerthan a second AC current time threshold.

In one embodiment, the apparatus includes a filter module that separateseach of the DC component and the one or more AC components from eachother. In a further embodiment, the filter module further substantiallyremoves a power source fundamental AC component and harmonics of thepower source fundamental AC component from within the one or more ACcomponents of the current monitored by the current monitor module. Inanother embodiment, the one or more AC components include three ACcomponents, and a frequency of each AC component of the three ACcomponents includes a frequency that is not a harmonic of a frequency ofthe other AC components and is not a harmonic of a fundamental frequencyof the power source. In another embodiment, the frequencies of the threeAC components include 85 hertz (“Hz”), 208 Hz, and 990 kilohertz(“kHz”).

In one embodiment, the apparatus includes a return current module thatdetermines a current in the grounding conductor, and a return currentthreshold module that determines if the current in the groundingconductor is below a return current threshold, where the trip modulefurther opens the contact in response to the return current thresholdmodule determining that the current in the grounding conductor is belowthe return current threshold for a period longer than a return currenttime threshold. In another embodiment, the apparatus includes a groundfault current module that determines a current in a connection between aground of the power source and a chassis ground of the power source, anda ground fault threshold module that determines if the currentdetermined by the ground fault module is above a ground fault threshold,where the trip module further opens the contact in response to theground fault threshold module determining that the current determined bythe ground fault current module is above the ground fault threshold fora period longer than a ground fault time threshold.

In one embodiment, the DC current minimum threshold corresponds to acombined pilot conductor and grounding conductor resistance of less than50 ohms. In another embodiment, the DC detection module isolates a DCcomponent of the current monitored by the current monitor module and theAC detection module determines the AC current corresponding to eachfrequency of the one or more AC components present in the currentmonitored by the current monitor module by isolating each AC currentwith a frequency of the one or more AC components.

A system monitoring ground current includes a power source and a groundmonitor in the power source. The ground monitor includes a signalgeneration module that injects a signal in a pilot conductor. The signalincludes a direct current (“DC”) component and one or more alternatingcurrent (“AC”) components. Each AC component of the one or more ACcomponents has a frequency different from other AC components of the oneor more AC components. Current in the pilot conductor is injected in agrounding conductor of a set of power cables connecting the power sourceto a load, and the grounding conductor connected to a ground return. Theground monitor, in one embodiment, includes a current monitor modulethat monitors current in at least one of the pilot conductor and theground return.

The ground monitor includes, in one embodiment, a DC detection modulethat determines a DC current present in the current monitored by thecurrent monitor module and an AC detection module that determines an ACcurrent corresponding to each frequency of the one or more AC componentspresent in the current monitored by the current monitor module Theground monitor includes, in one embodiment, a DC minimum thresholdmodule that determines if the DC current is below a DC current minimumthreshold and an AC threshold module that determines if one or more ofthe AC currents determined by the AC detection module is below an ACthreshold. The ground monitor includes a trip module that opens acontact in response to the DC threshold module determining that the DCcurrent is below the DC current minimum threshold longer than a DCcurrent minimum time threshold and/or the AC threshold moduledetermining that at least one of the AC currents is below an ACthreshold for longer than an AC current time threshold, the contactdisconnecting the power source from the set of power cables.

A method for ground monitor current sensing includes injecting a signalin a pilot conductor. The signal includes a direct current (“DC”)component and one or more alternating current (“AC”) components, whereeach AC component of the one or more AC components has a frequencydifferent from other AC components of the one or more AC components.Current in the pilot conductor is injected in a grounding conductor of aset of power cables connecting a power source to a load. The groundingconductor is connected to a ground return. The method includesmonitoring current in at least one of the pilot conductor and the groundreturn, determining a DC current present in the monitored current in oneor more of the pilot conductor and the ground return, and determining anAC current corresponding to each frequency of the one or more ACcomponents present in the current monitored in one or more of the pilotconductor and the ground return. The method includes, in one embodiment,determining if the DC current is below a DC current minimum thresholdand determining if one or more of the AC currents corresponding to afrequency of the one or more AC components in the monitored current isbelow an AC threshold. The method includes opening a contact in responseto one or more of determining that the DC current is below the DCcurrent minimum threshold longer than a DC current minimum timethreshold and determining that at least one of the AC currents is belowan AC threshold for longer than an AC current time threshold, thecontact disconnecting the power source from the set of power cables.

In one embodiment, the method includes determining if the DC current isabove a DC current maximum threshold and includes opening the contact inresponse to determining that the DC current is above the DC maximumcurrent threshold. In another embodiment, the DC current maximumthreshold corresponds to a current that is above a DC current in thepilot conductor or ground return that is at a level indicative of anoperating condition without a stray DC current component.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the invention will be readilyunderstood, a more particular description of the invention brieflydescribed above will be rendered by reference to specific embodimentsthat are illustrated in the appended drawings. Understanding that thesedrawings depict only typical embodiments of the invention and are nottherefore to be considered to be limiting of its scope, the inventionwill be described and explained with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIG. 1 is a schematic block diagram illustrating one embodiment of asystem for ground monitor current sensing in accordance with anembodiment of the present invention;

FIG. 2 is a schematic block diagram illustrating one embodiment of anapparatus for ground monitor current sensing in accordance with anembodiment of the present invention;

FIG. 3 is a schematic block diagram illustrating another embodiment ofan apparatus for ground monitor current sensing in accordance with anembodiment of the present invention;

FIG. 4 is a schematic block diagram illustrating one embodiment of amethod for ground monitor current sensing in accordance with anembodiment of the present invention;

FIG. 5 is a schematic block diagram illustrating another embodiment of amethod for ground monitor current sensing in accordance with anembodiment of the present invention;

FIG. 6 is a schematic block diagram illustrating one embodiment of awatchdog apparatus in accordance with an embodiment of the presentinvention;

FIG. 7 is a schematic block diagram illustrating another embodiment of awatchdog apparatus in accordance with an embodiment of the presentinvention;

FIG. 8 is a schematic block diagram illustrating a third embodiment of awatchdog apparatus in accordance with an embodiment of the presentinvention;

FIG. 9 is a schematic block diagram illustrating one embodiment of amethod for a watchdog function in accordance with an embodiment of thepresent invention;

FIG. 10 is a schematic block diagram illustrating another embodiment ofa method for a watchdog function in accordance with an embodiment of thepresent invention; and

FIG. 11 is a schematic block diagram illustrating a third embodiment ofa method for a watchdog function in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment. Thus, appearances of the phrases“in one embodiment,” “in an embodiment,” and similar language throughoutthis specification may, but do not necessarily, all refer to the sameembodiment, but mean “one or more but not all embodiments” unlessexpressly specified otherwise. The terms “including,” “comprising,”“having,” and variations thereof mean “including but not limited to”unless expressly specified otherwise. An enumerated listing of itemsdoes not imply that any or all of the items are mutually exclusiveand/or mutually inclusive, unless expressly specified otherwise. Theterms “a,” “an,” and “the” also refer to “one or more” unless expresslyspecified otherwise.

Furthermore, the described features, advantages, and characteristics ofthe embodiments may be combined in any suitable manner. One skilled inthe relevant art will recognize that the embodiments may be practicedwithout one or more of the specific features or advantages of aparticular embodiment. In other instances, additional features andadvantages may be recognized in certain embodiments that may not bepresent in all embodiments.

These features and advantages of the embodiments will become more fullyapparent from the following description and appended claims, or may belearned by the practice of embodiments as set forth hereinafter. As willbe appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method, and/or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module,” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having program code embodied thereon.

Many of the functional units described in this specification have beenlabeled as modules, in order to more particularly emphasize theirimplementation independence. For example, a module may be implemented asa hardware circuit comprising custom VLSI circuits or gate arrays,off-the-shelf semiconductors such as logic chips, transistors, or otherdiscrete components. A module may also be implemented in programmablehardware devices such as field programmable gate arrays, programmablearray logic, programmable logic devices or the like.

Modules may also be implemented in software for execution by varioustypes of processors. An identified module of program code may, forinstance, comprise one or more physical or logical blocks of computerinstructions which may, for instance, be organized as an object,procedure, or function. Nevertheless, the executables of an identifiedmodule need not be physically located together, but may comprisedisparate instructions stored in different locations which, when joinedlogically together, comprise the module and achieve the stated purposefor the module.

Indeed, a module of program code may be a single instruction, or manyinstructions, and may even be distributed over several different codesegments, among different programs, and across several memory devices.Similarly, operational data may be identified and illustrated hereinwithin modules, and may be embodied in any suitable form and organizedwithin any suitable type of data structure. The operational data may becollected as a single data set, or may be distributed over differentlocations including over different storage devices, and may exist, atleast partially, merely as electronic signals on a system or network.Where a module or portions of a module are implemented in software, theprogram code may be stored and/or propagated on in one or more computerreadable medium(s).

The computer readable medium may be a tangible computer readable storagemedium storing the program code. The computer readable storage mediummay be, for example, but not limited to, an electronic, magnetic,optical, electromagnetic, infrared, holographic, micromechanical, orsemiconductor system, apparatus, or device, or any suitable combinationof the foregoing.

More specific examples of the computer readable storage medium mayinclude but are not limited to a portable computer diskette, a harddisk, a random access memory (RAM), a read-only memory (ROM), anerasable programmable read-only memory (EPROM or Flash memory), aportable compact disc read-only memory (CD-ROM), a digital versatiledisc (DVD), an optical storage device, a magnetic storage device, aholographic storage medium, a micromechanical storage device, or anysuitable combination of the foregoing. In the context of this document,a computer readable storage medium may be any tangible medium that cancontain, and/or store program code for use by and/or in connection withan instruction execution system, apparatus, or device.

The computer readable medium may also be a computer readable signalmedium. A computer readable signal medium may include a propagated datasignal with program code embodied therein, for example, in baseband oras part of a carrier wave. Such a propagated signal may take any of avariety of forms, including, but not limited to, electrical,electro-magnetic, magnetic, optical, or any suitable combinationthereof. A computer readable signal medium may be any computer readablemedium that is not a computer readable storage medium and that cancommunicate, propagate, or transport program code for use by or inconnection with an instruction execution system, apparatus, or device.Program code embodied on a computer readable signal medium may betransmitted using any appropriate medium, including but not limited towire-line, optical fiber, Radio Frequency (RF), or the like, or anysuitable combination of the foregoing

In one embodiment, the computer readable medium may comprise acombination of one or more computer readable storage mediums and one ormore computer readable signal mediums. For example, program code may beboth propagated as an electro-magnetic signal through a fiber opticcable for execution by a processor and stored on RAM storage device forexecution by the processor.

Program code for carrying out operations for aspects of the presentinvention may be written in any combination of one or more programminglanguages, including an object oriented programming language such asJava, Smalltalk, C++, PHP or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

The computer program product may be shared, simultaneously servingmultiple customers in a flexible, automated fashion. The computerprogram product may be standardized, requiring little customization andscalable, providing capacity on demand in a pay-as-you-go model. Thecomputer program product may be stored on a shared file systemaccessible from one or more servers.

The computer program product may be integrated into a client, server andnetwork environment by providing for the computer program product tocoexist with applications, operating systems and network operatingsystems software and then installing the computer program product on theclients and servers in the environment where the computer programproduct will function.

In one embodiment software is identified on the clients and serversincluding the network operating system where the computer programproduct will be deployed that are required by the computer programproduct or that work in conjunction with the computer program product.This includes the network operating system that is software thatenhances a basic operating system by adding networking features.

Furthermore, the described features, structures, or characteristics ofthe embodiments may be combined in any suitable manner. In the followingdescription, numerous specific details are provided, such as examples ofprogramming, software modules, user selections, network transactions,database queries, database structures, hardware modules, hardwarecircuits, hardware chips, etc., to provide a thorough understanding ofembodiments. One skilled in the relevant art will recognize, however,that embodiments may be practiced without one or more of the specificdetails, or with other methods, components, materials, and so forth. Inother instances, well-known structures, materials, or operations are notshown or described in detail to avoid obscuring aspects of anembodiment.

Aspects of the embodiments are described below with reference toschematic flowchart diagrams and/or schematic block diagrams of methods,apparatuses, systems, and computer program products according toembodiments of the invention. It will be understood that each block ofthe schematic flowchart diagrams and/or schematic block diagrams, andcombinations of blocks in the schematic flowchart diagrams and/orschematic block diagrams, can be implemented by program code. Theprogram code may be provided to a processor of a general purposecomputer, special purpose computer, sequencer, or other programmabledata processing apparatus to produce a machine, such that theinstructions, which execute via the processor of the computer or otherprogrammable data processing apparatus, create means for implementingthe functions/acts specified in the schematic flowchart diagrams and/orschematic block diagrams block or blocks.

The program code may also be stored in a computer readable medium thatcan direct a computer, other programmable data processing apparatus, orother devices to function in a particular manner, such that theinstructions stored in the computer readable medium produce an articleof manufacture including instructions which implement the function/actspecified in the schematic flowchart diagrams and/or schematic blockdiagrams block or blocks.

The program code may also be loaded onto a computer, other programmabledata processing apparatus, or other devices to cause a series ofoperational steps to be performed on the computer, other programmableapparatus or other devices to produce a computer implemented processsuch that the program code which executed on the computer or otherprogrammable apparatus provide processes for implementing thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The schematic flowchart diagrams and/or schematic block diagrams in theFigures illustrate the architecture, functionality, and operation ofpossible implementations of apparatuses, systems, methods and computerprogram products according to various embodiments of the presentinvention. In this regard, each block in the schematic flowchartdiagrams and/or schematic block diagrams may represent a module,segment, or portion of code, which comprises one or more executableinstructions of the program code for implementing the specified logicalfunction(s).

It should also be noted that, in some alternative implementations, thefunctions noted in the block may occur out of the order noted in theFigures. For example, two blocks shown in succession may, in fact, beexecuted substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved. Other steps and methods may be conceived that are equivalentin function, logic, or effect to one or more blocks, or portionsthereof, of the illustrated Figures.

Although various arrow types and line types may be employed in theflowchart and/or block diagrams, they are understood not to limit thescope of the corresponding embodiments. Indeed, some arrows or otherconnectors may be used to indicate only the logical flow of the depictedembodiment. For instance, an arrow may indicate a waiting or monitoringperiod of unspecified duration between enumerated steps of the depictedembodiment. It will also be noted that each block of the block diagramsand/or flowchart diagrams, and combinations of blocks in the blockdiagrams and/or flowchart diagrams, can be implemented by specialpurpose hardware-based systems that perform the specified functions oracts, or combinations of special purpose hardware and program code.

FIG. 1 is a schematic block diagram illustrating one embodiment of asystem 100 for ground monitor current sensing in accordance with anembodiment of the present invention. The system 100 includes a groundmonitor apparatus 102, a power source 104, a three-phase power source106, a relay 108, a first phase 110, a second phase 112, a third phase114, a neutral grounding resistor 116, a grounding conductor 118, a load120 with a pilot wire diode (“PWD”) 124, a ground wire device (“GWD”)126 with a return current sensor 128, a ground return 130, a groundfault relay (“GFR”) current transformer (“CT”) 132, and a watchdogapparatus 134, which are described below. The system 100 may be similarto the system 100 of U.S. patent application Ser. No. 13/906,807entitled “GROUND MONITOR CURRENT SENSING” and filed on May 31, 2013 forDale Curtis, et al., [hereinafter “the '807 patent application”] whichis incorporated herein by reference. The '807 patent applicationincludes a ground monitor apparatus 102 that is different than theground monitor apparatus 102 described herein, but has similarfunctionality in monitoring ground current.

The system 100 includes, in one embodiment, a ground monitor apparatus102 that monitors current in the grounding conductor 118, and isdescribed in more detail with regard to the apparatus 200 of FIG. 2, theapparatus 300 of FIG. 3. The power source 104, in one embodiment,includes a three-phase power source 106. In one example, the three-phasepower source 106 is a wye-connected source. The three-phase power source106, in one example, is grounded through a neutral grounding resistor116. In another embodiment, the three-phase power source 106 may be adelta-connected power source. In another embodiment, the power source104 includes another type of power source, such as a single-phase powersource or a direct current (“DC”) power source. The three-phase powersource 106 may include some type of a generator, or maybe wired toanother source (not shown). In another embodiment, the power source 104may include multiple power sources. In a particular embodiment, thepower source 104 may be configured for an application in a mine. Thepower source 104, in one embodiment, is configured for miningapplications and in another embodiment is configured to meetrequirements of the Mine Safety and Health Administration (“MSHA”). Oneof skill in the art will recognize other power sources 104 that mayinclude a ground monitor apparatus 102.

In one embodiment, the power source 104 includes a relay 108. As usedherein, contact and relay are used interchangeably. The relay 108 mayinclude a contact that may be normally open or normally closed andclosing or opening the contact may open the relay 108. In one example,the relay 108 is a three-phase contactor. In another example, the relay108 is a circuit breaker with the remote trip. The relay 108 includes acapability to be commanded open by the ground monitor apparatus 102. Therelay 108 may include a fault detection module (not shown) that sends atrip signal to the relay 108 on detection of an overcurrent condition.For example, a fault detection module may include current sensing and/orvoltage sensing circuits may indicate an overcurrent or fault condition.The fault detection module may have an inverse time characteristic orother common protective relaying characteristic and may coordinate withother overcurrent protection devices upstream and downstream. The relay108 may also be opened for other conditions, such as a manual command toopen.

In one example, the relay 108 also includes the capability to becommanded open by the ground monitor apparatus 102. For example, theground monitor apparatus 102 may send a command to open the relay 108upon detection of a ground problem. In another embodiment, or the powersource 104 includes a different type of power source, such as a singlephase source, the relay 108 may include a different number of poles. Inanother example, the relay 108 may be a three-pole contactor. One ofskill in the art will recognize other types of relays 108 that may beused in conjunction with the power source 104 that includes a groundmonitor apparatus 102.

In one embodiment, the power source 104 includes a relay 108. In oneexample, the relay 108 is a three-phase contactor. In another example,the relay 108 is a circuit breaker with the remote trip. The relay 108includes a capability to be commanded open by the ground monitorapparatus 102. The relay 108 may include a fault detection module (notshown) that sends a trip signal to the relay 108 on detection of anovercurrent condition. For example, the fault detection module mayinclude current sensing and/or voltage sensing circuits may indicate anovercurrent or fault condition. The fault detection module may have aninverse time characteristic or other common protective relayingcharacteristic and may coordinate with other overcurrent protectiondevices upstream and downstream. The relay 108 may also be opened forother conditions, such as a manual command to open.

In one example, the relay 108 also includes the capability to becommanded open by the ground monitor apparatus 102. For example, theground monitor apparatus 102 may send a command to open the relay 108upon detection of a ground problem, such as a ground current differentthan expected. In another embodiment, or the power source 104 includes adifferent type of power source, such as a single phase source, the relay108 may include a different number of poles. In another example, therelay 108 may be a three-pole contactor. One of skill in the art willrecognize other types of relays 108 that may be used in conjunction withthe power source 104 that includes a ground monitor apparatus 102.

In one embodiment, the system 100 includes a set of power cables wiredto a load 120 with a first phase 110, a second phase 112, a third phase114, and a grounding conductor 118. In other embodiments, the set ofpower cables may include a neutral conductor (grounded conductor) wiredto the load and/or the pilot conductor 122. For safety, it is desirableto determine if the grounding conductor 118 is properly connectedbetween the power source 104 and the load 120. For example, if thegrounding conductor 118 is disconnected, has failed, or in some way isnot properly connected between the power source 104 and the load 120,there may be a hidden danger within the system 100. For instance, if ashort circuit occurs and the grounding conductor 118 is not properlyconnected, current may flow through the ground from the load 120 to thepower source 104 and may generate an electric field within the groundthat may cause an electrical hazard for personnel. In one embodiment,the ground monitor apparatus 102 helps to ensure that the groundingconductor 118 is properly connected and functioning.

In one embodiment, the ground monitor apparatus 102 injects a signalinto the grounding conductor 118 to sense an undesirable condition, suchas a high impedance within the grounding conductor 118, a short betweenthe pilot conductor 122 and ground, or other failure in the groundingconductor 118 or in the ground monitor apparatus 102 and associatedcomponents.

In one embodiment, the pilot conductor 122 connects the ground monitorapparatus 102 to the PWD 124 located in the load 120. The PWD 124, inone embodiment, includes one or more diodes, and the pilot conductor 122is wired to the diodes. A terminal of the PWD 124 may be wired to achassis ground of the load 120. The grounding conductor 118, in oneembodiment, is also connected to the chassis ground of the load 120. Inone embodiment, the ground monitor apparatus 102 connects an alternatingcurrent (“AC”) source to the pilot conductor 122. The PWD 124 mayprovide a voltage drop which may be used by the ground monitor apparatus102 to determine a grounding problem. If the PWD 124 fails short, if thepilot conductor 122 is shorted to ground, the current signal changes andthe ground monitor apparatus 102 is able to detect the change and openthe relay 108.

In an alternate embodiment, the load 120 does not include a PWD 124 witha diode and the pilot conductor 122 connects to chassis ground of theload 120 and to the grounding conductor 118. In the embodiment, theground monitor apparatus 102 may use a different current sensing schemethan where the PWD 124 with a diode is included. In another embodiment,a DC voltage source injects current into the pilot conductor 122 and theload does not include a PWD 124 with a diode. In another embodiment, aDC voltage source injects current into the pilot conductor 122 and theload includes a PWD 124 with a diode. In the embodiment, the diode inthe PWD 124 may be a zener diode with the pilot conductor 122 connectedto the cathode the grounding conductor 118 connected to the anode of thezener diode. Various embodiments will be described below with respect tothe apparatuses 200, 300, 301 of FIGS. 2, 3A and 3B.

In one embodiment, the power source 104 includes a GWD 126. The GWD 126,in one embodiment, includes a return current sensor 128. In one example,the return current sensor 128 includes a current transformer that sensescurrent in the grounding conductor 118. In another example, thegrounding conductor 118 is connected to back to back diodes within theGWD 126, and the back-to-back diodes are also connected to the chassisground of the power source 104. The back-to-back diodes, in someembodiments, provide some signal isolation for monitoring injectedsignal in the grounding conductor 118 from the pilot conductor 122. Inaddition, the back-to-back diodes in the GWD 126 help to keep anyvoltage on the grounding conductor 118 to within a diode drop of thechassis ground. In another embodiment, a saturable coil may replace theback-to-back diodes.

The saturable coil, in one embodiment, may help keep voltage on thegrounding conductor 118 within a saturation voltage of the chassisground. In another embodiment, the GWD 126 does not include back-to-backdiodes but includes a return current sensor 128 and the groundingconductor 118 is connected to chassis ground of the power source 104. Inanother embodiment, the GWD 126 does not include back-to-back diodes ora return current sensor 128 and the grounding conductor 118 is connectedto chassis ground of the power source 104. One skilled in the art willrecognize that various other methods exist that can help limit thevoltage potential between the grounding conductor 118 and the chassisground, while providing some signal isolation for monitoring injectedsignal in the grounding conductor 118. In one embodiment, the system 100includes a watchdog apparatus 134 in the ground monitor apparatus 102.The watchdog apparatus 134, in one embodiment, determines if the groundmonitor apparatus 102 is functioning properly. The watchdog apparatus134 is discussed in more detail in relation to the apparatuses 600, 700,800 of FIGS. 6-8.

FIG. 2 is a schematic block diagram illustrating one embodiment of anapparatus 200 for ground monitor current sensing in accordance with anembodiment of the present invention. The apparatus 200 includes a pilotconductor 122, a ground return 130, and a connection to a contact of arelay 108, which are substantially similar to those described above inrelation to the system 100 of FIG. 1. The apparatus 200 includes oneembodiment of a ground monitor apparatus 102 with a signal generationmodule 202, a current monitor module 204, a direct current (“DC”)detection module 206, an alternating current (“AC”) detection module208, a DC minimum threshold module 210, an AC threshold module 212, anda trip module 214, which are described below.

The apparatus 200 includes an embodiment of a signal generation module202 that that injects a signal in the pilot conductor 122. In oneembodiment, the signal generation module 202 connects to the pilotconductor 122 and ground return 130. The signal generated by the signalgeneration module 202 includes a DC component and one or more ACcomponents. The DC component may include a DC voltage that is higherthan a peak voltage of the one or more AC components. For example, if anAC component includes a sinusoidal waveform with a peak voltage of 5volts (“V”), the DC component may be 7 V. In another embodiment, the DCcomponent is zero volts or substantially zero volts. For example, the DCcomponent may be 0.1 V, 0.5 V, or other voltage that is substantiallyzero or close to zero or a voltage substantially less than a peakvoltage of an AC component.

Each AC component of the one or more AC components includes a frequencydifferent from other AC components of the one or more AC components.Current in the pilot conductor 122 is injected in the groundingconductor of the set of power cables connecting the power source 104 tothe load 120 and the grounding conductor 118 of the set of power cablesis connected to the ground return 130. In one embodiment, the one ormore AC components are sinusoidal waveforms of different frequencies. Inone embodiment, the one or more AC components are three AC components ofdifferent frequencies. In another embodiment, four or more AC componentsare generated by the signal generation module 202.

In one embodiment, each of the AC components generated by the signalgeneration module 202 has a fundamental frequency that is not a harmonicof the fundamental frequency of an AC voltage generated by the powersource 104. For example, the power source 104 may generate 60 hertz(“Hz”), 120 Hz, 400 Hz, etc. Where the power source 104 produces powerat 60 Hz, the AC components generated by the signal generation module202 may be 85 Hz, 208 Hz, and 990 Hz, which are not harmonic frequenciesof 60 Hz. In one embodiment, the AC components have a fundamentalfrequency that differs significantly from harmonic frequencies of thefundamental frequency of the power source 104.

The apparatus 200 includes, in one embodiment, a current monitor module204 that monitors current in at least the pilot conductor 122 or theground return 130 or both. For example, the current monitor module 204may monitor current in the pilot conductor 122. In another embodiment,the current monitor module 204 monitors current in the ground return130. In one embodiment, in normal situations current in the pilotconductor 122 is substantially the same as current in the ground return130. In another embodiment, the current monitor module 204 monitorscurrent in both the pilot conductor 122 and ground return 130, forexample for redundancy. The current monitor module 204 may use a HallEffect current sensor, a current transformer, a resistor, or othercurrent sensing method known to those of skill in the art. In oneembodiment, the current monitor module 204 includes current sensingcapable of monitoring AC and DC current. In one embodiment, the currentmonitor module 204 creates a voltage representative of current in thepilot conductor 122 or ground return 130. When referring to currentmonitored by the current monitor module 204, one of skill in the artwill recognize that the current monitor module 204 may use a voltagesignal and modules using the current monitored by the current monitormodule 204 may use one or more voltage signals representing current inthe pilot conductor 122/ground return 130. Thus the current monitormodule 204 includes a capability to monitor the DC component and the oneor more AC components.

The apparatus 200, in one embodiment, includes a DC detection module 206that determines a DC current present in the current monitored by thecurrent monitor module 204 and an AC detection module 208 thatdetermines an AC current corresponding to each frequency of the one ormore AC components present in the current monitored by the currentmonitor module 204. For example, the DC detection module 206 maydetermine a DC current present in the pilot conductor 122 or groundreturn 130. In one embodiment, the DC detection module 206 includesfiltering to filter DC current from the current monitored by the currentmonitor module 204. For example, the DC detection module 206 may includea low pass filter that substantially removes AC content from the currentmonitored by the current monitor module 204. One of skill in the artwill recognize other ways for the DC detection module 206 to determineDC current present in the current monitored by the current monitormodule 204.

In one embodiment, The AC detection module 208 determines an AC currentcorresponding to each frequency of the one or more AC components presentin the current monitored by the current monitor module 204 by filteringthe DC component from the monitored current and detecting each frequencyseparately. For example, the AC detection module 208 may include filtersto determine each frequency. In one embodiment, the AC detection module208 uses one or more comb filters tuned to the frequencies of the ACcomponents. In another embodiment, the AC detection module 208 uses oneor more band pass filters that pass the frequencies of the AC componentswhile eliminating other frequencies. In another embodiment, the ACdetection module 208 uses one or more notch filters. In anotherembodiment, the AC detection module 208 uses a combination of the abovementioned filters. For example, the AC detection module 208 may use acomb filter tuned to each frequency of the AC components and may theninclude band pass filters and notch filters to further isolate thefrequencies of the AC components. Resulting waveforms may then have aspecific amplitude for each. One of skill in the art will recognizeother ways for the AC detection module 208 to determine an AC current,which may be represented as a voltage, corresponding to each frequencyof the one or more AC components.

The apparatus 200, in one embodiment, includes a DC minimum thresholdmodule 210 that determines if the DC current is below a DC currentminimum threshold. For example, the DC minimum threshold module 210 mayinclude a comparator that compares the DC current determined by the DCdetection module 206 with a DC current minimum threshold. The DC minimumthreshold module 210 may use other circuits as well to determine of theDC current is below a DC current minimum threshold. In anotherembodiment, the DC minimum threshold module 210 may include redundantcircuitry and may determine if the DC current is below a first DCcurrent minimum threshold and a second DC current minimum threshold. Thefirst and the second DC minimum thresholds may be the same or different.Redundancy may be used to increase reliability.

In one embodiment, the DC current minimum threshold corresponds to acombined pilot conductor 122 and grounding conductor 118 resistance ofless than 50 ohms. In another embodiment, the DC current minimumthreshold corresponds to a combined pilot conductor 122, groundingconductor 118, and ground return 130 resistance of less than 50 ohms.Fifty ohms corresponds to an MSHA requirement and the DC current minimumthreshold may be set to correspond to 50 ohms or less. For example, ifthe DC component is a particular voltage, the DC current minimumthreshold may correspond to the DC component voltage divided by 50 ohmsor a lower resistance. In one embodiment, the trip module 214 opens thecontact 108 when the DC current is below the DC minimum currentthreshold such that if resistance of the pilot conductor 122, groundingconductor 118, and ground return exceeds a value of 50 ohms or someresistance value just under 50 ohms, the trip module 214 opens thecontact 108. A first DC current minimum threshold may correspond to 45ohms and a second DC current minimum threshold may correspond to 48ohms.

In another embodiment, the apparatus 200 includes an AC threshold module212 that determines if one or more of the AC currents determined by theAC detection module is below an AC threshold. In one embodiment, the ACthreshold module 212 uses a single AC threshold for each AC currentcorresponding to an AC component. In another embodiment, the ACthreshold module 212 uses a different AC threshold for each AC currentcorresponding to an AC component. Note that some of the AC thresholdsfor the AC currents for the various AC components may be the same.

In one embodiment, the AC threshold module 212 uses a peak current tocompare to an AC threshold. In another embodiment, the AC thresholdmodule 212 uses a root-mean-square (“RMS”) of the current to compare toan AC threshold. The AC threshold module 212 may use one or morecomparators or circuits with a similar function to determine if one ormore of the AC currents is below one or more AC thresholds. In anotherembodiment, the AC current may be digitized and the AC threshold module212 may digitally compare one or more of the AC currents to one or moreAC thresholds. In another embodiment, the AC threshold module 212includes redundant circuitry so that for each current corresponding toan AC component, there are at least two comparators or similarcircuitry. One of skill in the art will recognize other ways for the ACthreshold module 212 to determine if one or more of the AC currents isbelow an AC threshold.

The apparatus 200, in one embodiment, includes a trip module 214 thatopens a contact 108 in response to the DC threshold module determiningthat the DC current is below the DC current minimum threshold longerthan a DC current minimum time threshold and/or the AC threshold moduledetermining that at least one of the AC currents is below an ACthreshold for longer than an AC current time threshold. The contact 108disconnects the power source 104 from the set of power cables. Note thatuse of the phrase “opens a contact 108” may include closing a normallyopen contact and/or opening a normally closed contact that is part ofthe relay 108 such that the power source 104 is disconnected from theset of power cables to the load 120.

The DC current minimum time threshold may be zero or substantially zeroor may be an appreciable amount of time. For example, the DC currentminimum time threshold may be set to zero while a time may elapsebetween sending a signal and actual opening due to typical circuit delaytime while the delay is unintentional. In another embodiment, the DCcurrent minimum time threshold may be set to a value that accounts fortransient conditions, motor start times, downstream overcurrent devicereaction time, or other delay known to those of skill in the art.Likewise the AC current time threshold may be zero, substantially zero,or another time and each AC component may have the same or a differentAC current time threshold.

In one embodiment, the trip module 214 opens the contact 108 in responseto one of the DC threshold module determining that the DC current isbelow the DC current minimum threshold longer than a DC current minimumtime threshold and the AC threshold module determining that an ACcurrent is below an AC threshold for longer than an AC current timethreshold. In another embodiment, the trip module 214 opens the contact108 in response to some combination of the DC threshold moduledetermining that the DC current is below the DC current minimumthreshold longer than a DC current minimum time threshold and the ACthreshold module determining that at least one or more of the ACcurrents is below an AC threshold for longer than an AC current timethreshold.

In one embodiment, the apparatus 200 includes multiple DC currentminimum thresholds and multiple AC current minimum thresholds for eachAC component and the trip module 214 reacts differently depending uponwhich threshold or thresholds are crossed. For example, certain AC or DCcurrent minimum thresholds may cause the trip module 214 to open thecontact 108 without any other condition where other AC or DC currentminimum thresholds may require some combination of threshold crossingsto open. In other embodiments, certain AC or DC current minimumthresholds may merely trigger an alert while others may cause the tripmodule 214 to open the contact 108.

In one embodiment, the trip module 214 opens the contact 108 in responseto the AC threshold module 212 determining that for at least two of theAC currents, an AC current is below an AC threshold for longer than afirst AC current time threshold. In a further embodiment, the signalgeneration module 202 generates three AC components and the trip module214 further opens the contact 108 in response to the AC threshold module212 determining that each of the three AC currents is below an ACthreshold for longer than a second AC current time threshold. In oneembodiment, the first AC current time threshold is longer than thesecond AC current time threshold. For example, the first AC current timethreshold may be 250 milliseconds (“mS”) and the second AC current timethreshold may be 150 mS.

FIG. 3 is a schematic block diagram illustrating another embodiment ofan apparatus 300 for ground monitor current sensing in accordance withan embodiment of the present invention. The apparatus 300 includes apilot conductor 122, a ground return 130, a connection to a contact of arelay 108, a grounding conductor 118, a return current sensor 128, and aGFR CT 132, which are substantially similar to those described above inrelation to the system 100 of FIG. 1 and one embodiment of a groundmonitor apparatus 102 with a signal generation module 202, a currentmonitor module 204, a direct current (“DC”) detection module 206, analternating current (“AC”) detection module 208, a DC minimum thresholdmodule 210, an AC threshold module 212, and a trip module 214, which aresubstantially similar to those described in relation to the apparatus200 of FIG. 2. The apparatus 300 includes, in various embodiments, a DCmaximum threshold module 302, a return current module 304, a returncurrent threshold module 306, a ground fault current module 308, and aground fault threshold module 310, which are described below.

The apparatus 300, in one embodiment, includes a DC maximum thresholdmodule 302 that determines if the DC current, determined by the DCdetection module 206, is above a DC current maximum threshold. In oneembodiment, the trip module 214 also opens the contact 108 in responseto the DC maximum threshold module 302 determining that the DC currentis above the DC maximum current threshold. In some circumstances, thegrounding conductor 118 or other circuit of the system 100 may includestray DC voltage and/or current that may increase the DC currentcomponent of the current in the pilot conductor 122 or ground return 130monitored by the current monitor module 204 and the DC detection module206 may determine that the DC current is above a DC current maximumthreshold. In one example, the DC current maximum threshold is set at alevel that corresponds to a current that is above a DC current in thepilot conductor 122 or ground return 130 that is at a level indicativeof an operating condition without a stray DC current component.

In one embodiment, the PWD 124 includes one or more diodes such that thePWD 124 has a particular voltage drop when the signal is applied to thepilot conductor 122. For example, the PWD 124 may include a zener diodewith a specific breakdown voltage. In another example, the PWD 124includes one diode or two or more diodes in series oriented to conductcurrent injected in the pilot conductor 122. The diodes include avoltage drop. A number of diodes may be used to create a desired voltagedrop across the PWD 124 during normal operation. In one embodiment, theDC current maximum threshold corresponds to a current that is above a DCcurrent in the pilot conductor 122 or ground return 130 when the load120 includes a properly functioning PWD 124. For example, if the voltagedrop across the PWD 124 is 5 V, the DC current maximum threshold may be7 V.

The apparatus 300, in one embodiment, includes a return current module304 that determines a current in the grounding conductor 118, and areturn current threshold module 306 that determines if the current inthe grounding conductor 118 is below a return current threshold. In theembodiment, the trip module 214 also opens the contact 108 in responseto the return current threshold module 306 determining that the currentin the grounding conductor 118 is below the return current threshold fora period longer than a return current time threshold. In one example,the return current module 304 includes a return current sensor 128positioned to determine current in the grounding conductor 118.

The return current sensor 128, in one embodiment, is a currenttransformer. The return current sensor 128, in one embodiment, is in theGWD 126 and monitors current before the grounding conductor 118 connectsto one or more diodes connected to a chassis ground. In anotherembodiment, return current sensor 128 is a Hall-Effect sensor. Inanother embodiment, the return current sensor 128 includes a resistorand voltage across the resistor is proportional to current in thegrounding conductor 118. One of skill in the art will recognize othertypes of return current sensors 128 capable of measuring current in thegrounding conductor 118. The return current module 304 may also includeother circuitry, such as resistors, capacitors, op amps, etc. known tothose of skill in the art.

In one embodiment, the return current threshold may be a value thatrepresents current just below a current level for the signal injected inthe pilot conductor 122 during typical operation. In a circumstancewhere the signal generation module 202 injects a signal and current inthe grounding conductor 118 is below an expected current, which is abovethe return current threshold, this condition may represent a situationwhere there is a break or higher impedance somewhere in the circuitformed by the pilot conductor 122, the PWD 124 (where included), and thegrounding conductor 118 and all or a portion of the current of thesignal injected by the signal generation module 202 may be returningthrough an alternate ground path. In one embodiment, the return currenttime threshold is zero or substantially zero, for example where thereturn current time threshold is set to not include any intentionaldelay. In another embodiment, the return current threshold is set to avalue greater than zero. The return current threshold may be set to avalue longer than expected transients or other condition where openingthe contact 108 may be undesirable. One of skill in the art willrecognize other appropriate values for the return current timethreshold.

In one embodiment, the apparatus 300 includes a ground fault currentmodule 308 that determines a current in a connection between a ground ofthe power source 104 and a chassis ground of the power source 104. Forexample, the power source 104 may include a connection to chassis groundand the ground fault current module 308 may determine current in theconnection. In one embodiment, the ground connection of the power source104 may include a neutral grounding resistor 116. The ground faultcurrent module 308 may monitor current through the neutral groundingresistor 116 or in conductors between a ground of the power source 104and a chassis ground. In one embodiment, the chassis ground is also anearth ground. The neutral grounding resistor 116, in some embodiments,is sized to limit current during a fault condition to below a specifiedvalue. For example, where the power source 104 is for mining equipment,the neutral grounding resistor 116 may be sized to limit current duringa fault to a level specified by MSHA.

In one embodiment, the ground fault current module 308 measures currentby measuring voltage across the neutral grounding resistor 116. Inanother embodiment, the ground fault current module 308 measures currentusing a current transformer, such as the GFR CT 132 depicted in FIG. 1.In another embodiment, the ground fault current module 308 includesother circuitry to provide a signal useful to the ground fault thresholdmodule 310, such as a voltage proportional to current in the connectionbetween the ground of the power source 104 and the chassis ground.

The apparatus 300 includes, in one embodiment, a ground fault thresholdmodule 310 that determines if the current determined by the ground faultmodule 308 is above a ground fault threshold. The trip module 214 alsoopens the contact 108 in response to the ground fault threshold module310 determining that the current determined by the ground fault currentmodule 308 is above the ground fault threshold for a period longer thana ground fault time threshold. In one embodiment, the ground faultthreshold is zero or substantially zero, for example where the groundfault threshold is set to zero without any intentional delay. In anotherembodiment, the ground fault threshold is set to a value higher thanzero. The ground fault threshold may be used to coordinate with otherground fault trip settings of downstream overcurrent protection devices,may be set to account for acceptable transient conditions, etc. Thereturn current threshold module 306 and the ground fault thresholdmodule 310 may include comparators, op amps, etc. to compare a currentsignal with a threshold.

In one embodiment, the apparatus 300 includes a filter module 312 thatthat separates each of the DC component and the one or more ACcomponents from each other. For example, the filter module 312 mayinclude the DC detection module 206 and the AC detection module 208 andmay include a comb filter, band pass filters, etc. as mentioned above inrelation to the DC detection module 206 and the AC detection module 208.In one embodiment, the filter module 312 substantially removes a powersource fundamental AC component and harmonics of the power sourcefundamental AC component from within the one or more AC components ofthe current monitored by the current monitor module 204. For example,the fundamental AC component of the power source 104 may be 60 Hz andthe filter module 312 may remove 60 Hz and harmonics of 60 Hz.

FIG. 4 is a schematic block diagram illustrating one embodiment of amethod 400 for ground monitor current sensing in accordance with anembodiment of the present invention. The method 400 begins and injects402 a signal in a pilot conductor 122. The signal includes a DCcomponent and one or more AC components. Each AC component has afrequency different from other AC components. As depicted in FIG. 1,current in the pilot conductor 122 is injected in the groundingconductor 118 of a set of power cables connecting the power source 104to the load 120. Also, the grounding conductor 118 connects to theground return 130. In one embodiment, the signal generation module 202injects 402 the signal in the pilot conductor 122. The method 400monitors 404 current in the pilot conductor 122, the ground return 130,or both. The current monitor module 204, in one embodiment, monitors 404the current. The method 400 determines 406 a DC current present in themonitored current and determines 408 if the DC current is below a DCcurrent minimum threshold. In one embodiment, the DC detection module206 determines the DC current present in the current monitored 404 bythe current monitor module 204 and the DC minimum threshold module 210determines 408 if the DC current is below a DC current minimumthreshold.

If the method 400 determines 408 that the DC current is not below a DCcurrent minimum threshold, the method 400 returns and continues toinject 402 a signal in the pilot conductor 122. If the method 400determines 408 that the DC current is below a DC current minimumthreshold, the method 400 opens 410 the contact 108, and the method 400ends. The method 400 determines 412 an AC current corresponding to eachfrequency of the one or more AC components present in the monitoredcurrent and determines 414 if one or more of the AC currents is below anAC threshold. If the method 400 determines 414 that one or more of theAC currents is not below an AC threshold, the method 400 returns andcontinues to inject 402 a signal in the pilot conductor 122. If themethod 400 determines 414 that one or more of the AC currents is belowan AC threshold, the method 400 opens 410 the contact 108, and themethod 400 ends. The DC detection module 206, the AC detection module208, the DC minimum threshold module 210, and the AC threshold module212 may be used in determining DC and AC components and if the DC and ACcomponents are below thresholds.

FIG. 5 is a schematic block diagram illustrating another embodiment of amethod 500 for ground monitor current sensing in accordance with anembodiment of the present invention. The method 500 begins and injects502 a signal in a pilot conductor 122. The signal includes a DCcomponent and one or more AC components. Each AC component has afrequency different from other AC components. The method 500 monitors504 current in the pilot conductor 122, the ground return 130, or both.The method 500 determines 506 a DC current present in the monitoredcurrent and determines 508 if the DC current is below a DC currentminimum threshold. If the method 500 determines 508 that the DC currentis below a DC current minimum threshold, the method 500 opens 512 thecontact 108. If the method 500 determines 508 that the DC current is notbelow a DC current minimum threshold, the method 500 determines 510 ifthe DC current is above a DC current maximum threshold. If the method500 determines 510 that the DC current is not above the DC currentmaximum threshold, the method 500 returns and continues to inject 502 asignal in the pilot conductor 122. If the method 500 determines 510 thatthe DC current is above the DC current maximum threshold, the method 500opens 512 the contact 108. The DC maximum threshold module 302 may beused to determine 510 if the DC current is above the DC current maximumthreshold.

The method 500 determines 514 an AC current corresponding to eachfrequency of the one or more AC components present in the monitoredcurrent and determines 516 if one or more of the AC currents is below anAC threshold. If the method 500 determines 516 that one or more of theAC currents is not below an AC threshold, the method 500 returns andcontinues to inject 502 a signal in the pilot conductor 122. If themethod 500 determines 516 that one or more of the AC currents is belowan AC threshold, the method 500 opens 512 the contact 108.

The method 500 determines 518 a current in the grounding conductor 118and determines 520 if the current in the grounding conductor 118 isbelow a return current threshold. If the method 500 determines 520 thatthe current in the grounding conductor 118 is not below the returncurrent threshold, the method 500 returns and continues to inject 502 asignal in the pilot conductor 122. If the method 500 determines 520 thatthe current in the grounding conductor 118 is below the return currentthreshold, the method 500 opens 512 the contact 108. The return currentmodule 304 and the return current threshold module 306 may be used todetermine 518 current in the grounding conductor 118 and to determine520 if the current in the grounding conductor 118 is below a returncurrent threshold.

The method 500 determines 522 a current in a connection between a groundof the power source 104 and a chassis ground of the power source 104 anddetermines 524 if the ground current is above a ground fault threshold.If the method 500 determines 524 that the ground current is not above aground fault threshold, the method 500 returns and continues to inject502 a signal in the pilot conductor 122. If the method 500 determines524 that the ground current is above a ground fault threshold, themethod 500 opens 512 the contact 108, and the method 500 ends. Theground fault current module 308 and the ground fault threshold module310 may be used to determine 522 a current in the connection between theground of the power source 104 and the chassis ground of the powersource 104 and to determine 524 if the ground current is above a groundfault threshold.

FIG. 6 is a schematic block diagram illustrating one embodiment 600 of awatchdog apparatus 134 in accordance with an embodiment of the presentinvention. The watchdog apparatus 134 includes a watchdog signal module602, a signal monitoring module 604, and an alert module 606, which aredescribed below. In one embodiment, the watchdog apparatus 134determines if a device, such as a microprocessor or a device capable ofgenerating a signal, is functioning properly. The device may be part ofa computing device, a smartphone, an appliance, or any other devicewhere monitoring functionality of a microprocessor or similar devicewould be beneficial. In one embodiment, the watchdog apparatus 134 ispart of the ground monitor apparatus 102, and the watchdog apparatus 134monitors functionality of the ground monitor apparatus 102. In oneembodiment, the ground monitor apparatus 102 with the watchdog apparatus134 complies with MSHA testing standard 30 the Code of FederalRegulations (“C.F.R.”) section 75, 30 C.F.R. section 77, AmericanSociety for Testing and Materials (“ASTP”) standard 2135, and/or MSHAdocument number ACRI 2003. Typical ground monitors fail to comply withthese standards. The ground monitor apparatus 102 described herein isdesigned to meet these standards.

In one embodiment, the watchdog apparatus 134 includes a watchdog signalmodule 602 that generates a watchdog signal. The watchdog signal isgenerated at a specific interval and includes an identifiablecharacteristic. The watchdog signal module is part of a device beingmonitored for functionality. For example, the identifiablecharacteristic may be a pulse with a particular pulse width. The pulsemay occur at a specific interval or frequency. In one embodiment, eachpulse includes the same identifiable characteristic. In another example,the identifiable characteristic is a digital pattern and the digitalpattern is generated periodically.

In another embodiment, the identifiable characteristic includes a pulsethat fits within a specific frequency range. Additional frequenciescould be included with a carrier frequency. In another embodiment, theidentifiable characteristic includes a pulse with a particular patternof highs and lows (i.e. ones and zeros) that is periodically broadcast.In another embodiment, the identifiable characteristic includes ananalog signal coming from a digital-to-analog converter (“DAC”) with aspecific, measurable characteristic, such as multiple tones at differentfrequencies. One of skill in the art will recognize other identifiablecharacteristics that may be included in a signal that may be used todetermine that a device is functioning.

In one embodiment, the watchdog signal module 602 is included within adevice being monitored while the signal monitoring module 604 isexternal to the monitored device but internal to a device controlledand/or powered by the monitored device. For example, the monitoreddevice may be a microprocessor in a computer and the watchdog signalmodule 602 is in the microprocessor while the signal monitoring module604 and possibly the alert module 606 is within the computer andexternal to the microprocessor. In another embodiment, the watchdogsignal module 602 is in a processor in the ground monitor apparatus 102while the signal monitoring module 604 and at least a portion of thealert module 606 are external to the processor but within the groundmonitor apparatus 102.

In one embodiment, the watchdog apparatus 134 includes a signalmonitoring module 604 that determines if the watchdog signal includesthe identifiable characteristic. For example, if the identifiablecharacteristic is a pulse of a specific width, the signal monitoringmodule 604 can determine if the pulse width is as expected. If thewatchdog signal includes specific frequencies, the signal monitoringmodule 604 can determine if the specific frequencies are present. If thewatchdog signal is a digital pattern, the signal monitoring module 604can determine if the digital pattern is present in the watchdog signal.Typically, if the monitored device is malfunctioning, there is a goodchance that the watchdog signal will vary from the identifiablecharacteristic.

In one embodiment, the watchdog apparatus 134 includes an alert module606 that sends and alert signal in response to the signal monitoringmodule 604 determining that the watchdog signal does not include theidentifiable characteristic. The alert signal may include a message to aparticular device. In another embodiment, the alert signal includestriggering an action, such as a reset or a shutdown of the monitoreddevice or the device that includes the modules 602-606. In anotherembodiment, the alert signal is used to trigger an action external tothe device that includes the modules 602-606. For example, if the deviceis the ground monitor apparatus 102, the alert signal may be a tripsignal that opens the contact 108 to disconnect the power source 104from the load 120. In another embodiment, the alert signal may includeone or more messages, notifications, etc. as well as triggering someaction like opening the contact 108 or shutting down a processor.

FIG. 7 is a schematic block diagram illustrating another embodiment 700of a watchdog apparatus 134 in accordance with an embodiment of thepresent invention. The embodiment 700 of the watchdog apparatus 134 in aground monitor apparatus 102 and the watchdog apparatus 134 includes awatchdog signal module 602 with a first pulse module 702, a signalmonitoring module 604 with a first pulse average module 704 and a firstpulse window module 706, and an alert module 606, which is substantiallysimilar to the alert module 606 described in relationship with theembodiment 600 of FIG. 6. In various embodiments, the watchdog apparatus134 may also include a second pulse module 708, a second pulse averagemodule 710, and a second pulse window module 712, which are describedbelow.

In one embodiment, an embodiment 700 of the watchdog apparatus 134includes a watchdog signal module 602 with a first pulse module 702 thatgenerates a first pulse signal of a first frequency. The first pulsesignal includes a pulse of a first pulse width. The watchdog signalincludes, in this embodiment, the first pulse signal. The first pulsemodule 702, in one embodiment, is included in a device that is beingmonitored by the watchdog apparatus 134. For example, the first pulsemodule 702 may be included in a processor. The processor, in oneembodiment, is in the ground monitor apparatus 102.

The first pulse includes a first pulse width that is typically less thanthe period of the first frequency. The first pulse module 702 generatesa first pulse at the first frequency. For example, if the firstfrequency is 10 kilohertz (“kHz”), the period for this frequency is 100microseconds and the first pulse module generates a first pulse every100 microseconds. The first pulse width is then less than 100microseconds. For example, if the first pulse width is 40 microseconds,an output of the first pulse module 702 may be high for 40 microsecondsand low for 60 microseconds for each period of the first frequency.

In one embodiment, the watchdog apparatus 134 includes a first pulseaverage module 704 that determines a first pulse average. The firstpulse average includes an average level of one or more periods of thefirst pulse signal. For example, where the first pulse is 40microseconds and the first frequency is 10 kHz, the first pulse averagemay be 40%. The first pulse average, in one embodiment, is expressed asa voltage. For example, if the first pulse is 5 V for 40 microsecondsand 0 V for 60 microseconds, the first pulse average may be 40% of 5 V,or 2 V. in other embodiments, other high and low voltages may be usedand the first pulse average may differ. One of skill in the art willrecognize other ways to average the first pulse signal. In oneembodiment, the first pulse average module 704 determines a first pulseaverage for each period of the first switching frequency. In anotherembodiment, the first pulse average module 704 averages two or morefirst pulse signals. Averaging more than one first pulse signals mayreduce variations due to transients and noise.

In another embodiment, the watchdog apparatus 134 includes a first pulsewindow module 706 that determines if the first pulse average is betweena first pulse maximum threshold and a first pulse minimum threshold. Forexample, where the first pulse average is 40%, the first pulse maximumthreshold may be 50% and the first pulse minimum threshold may be 30%.Where the first pulse average is 2 V, the first pulse maximum thresholdmay be 2.5 V and the first pulse minimum threshold may be 1.5V. Wherethe first pulse average is determined by the first pulse average module704 to be below 1.5V or above 2.5 V, the first pulse window module 706may determine that the first pulse average is outside of an allowablewindow of 1.5V to 2.5 V. The alert module 606 then sends an alert signalwhen the first pulse window module 706 determines that the first pulseaverage is not between the first pulse maximum threshold and the firstpulse minimum threshold.

In one embodiment, an embodiment 700 of the watchdog apparatus 134includes a second pulse module 708, a second pulse average module 710,and a second pulse window module 712. The second pulse module 708generates a second pulse signal of a second frequency and the secondpulse signal includes a pulse of a second pulse width. The second pulsemodule 708 generates a second pulse signal that is independent of thefirst pulse signal. In other words, the second pulse signal is generatedin a way to be separate from the first pulse signal, such as on adifferent pin of a processor than the first pulse signal. Having asecond pulse signal may provide a more fault tolerant design than anembodiment where only a first pulse signal is generated. For example, ifa pin on a processor where the first pulse signal is available isshorted, the second pulse signal may be available.

In one embodiment, the first pulse width is the same as the second pulsewidth. In another embodiment, the first pulse width and the second pulsewidth are different. In another embodiment, the second pulse width is acompliment or an inversion of the first pulse width. For example, if thefirst pulse width is 40 microseconds starting at the beginning of aperiod, the second pulse width may be 60 microseconds starting 40microseconds into the period. The first frequency may be the same ordifferent than the second frequency.

The second pulse average module 710 determines a second pulse averagethat includes an average level of one or more periods of the secondpulse signal, and the second pulse window module 712 determines if thesecond pulse average is between a second pulse maximum threshold and asecond pulse minimum threshold. The second pulse average module 710 andthe second pulse window module 712, in one embodiment, operate similarlyto the first pulse average module 704 and the second pulse window module712 except function with the second pulse signal. In one embodiment, thesecond pulse average module 710 and the second pulse window module 712operate independently from the first pulse average module 704 and thesecond pulse window module 712, which may increase fault tolerance.

FIG. 8 is a schematic block diagram illustrating a third embodiment 800of a watchdog apparatus 134 in accordance with an embodiment of thepresent invention. In the embodiment, the watchdog apparatus 134 is inthe ground monitor apparatus 102 and includes a watchdog signal module602 with a first pulse module 702 and a signal monitoring module 604with a first pulse average module 704 and a first pulse window module706, an alert module 606, a second pulse module 708, a second pulseaverage module 710, and a second pulse window module 712, which aresubstantially similar to those described in relation to the embodiment700 of the watchdog apparatus 134 in FIG. 7. The embodiment 800 of thewatchdog apparatus 134 also includes, in various embodiments, a tripmodule 802, a trip error module 806, a microprocessor 808, and a powersupply failure module 810, and the ground monitor apparatus 102 includesa fault detection circuits 804, which are described below.

In one embodiment, the watchdog apparatus 134 includes a trip module 802that generates a trip signal. The trip module 802 generates the tripsignal in response to a fault condition in the power source 104providing power to the load 120. For example, fault detection circuits804 in the ground monitor apparatus 102 or other overcurrent protectiondevice may detect a fault, such as unexpected current in the groundingconductor 118, less current or different current than expected,overcurrent, or other function where the ground monitor apparatus 102would generate a trip signal may cause the trip module 802 to generate atrip signal. In one embodiment, the trip module 802 of the apparatus 800of FIG. 8 is the same as the trip module 214 of the system 100 of FIG. 1and apparatuses 200, 300 of FIGS. 2 and 3. For example, the trip module214 of the system 100 of FIG. 1 and apparatuses 200, 300 of FIGS. 2 and3 may include the functionality of the trip module 802 of the apparatus800 of FIG. 8 along with functionality described in relation to thesystem 100 of FIG. 1 and apparatuses 200, 300 of FIGS. 2 and 3.

In the embodiment, the trip signal includes an active high signal and anactive low signal where the active high signal transitions from low tohigh when the trip module 802 generates the trip signal and the activelow signal transitions from high to low when the trip module 802generates the trip signal. The alert module 606 sends the alert signalin response to the active high signal transitioning high and/or theactive low signal transitioning low. Having an active high signal and anactive low signal may provide a more fault tolerant watchdog apparatus134.

In one embodiment, the watchdog apparatus 134 includes a trip errormodule 806 that sends a trip error signal in response to determiningthat the active high signal and the active low signal are bothsimultaneously high or both simultaneously low for a predeterminedamount of time. For example, if the active high signal is low when itshould be high and the active low signal is low, this condition maysignal a fault condition, such as the active high line of the tripsignal is shorted. Similarly, if the active high signal remains low whenit should transition high and the active low signal transitions low,then again both the active high and the active low signals are both low,which again may indicate a fault condition. Also if both trip signalsare simultaneously high, this again may indicate an unwanted condition.Any of these conditions where both the active high and the active lowsignals are the same causes the trip error module 806 to send a triperror signal.

In one embodiment, the trip error signal includes a communication thatthere is a problem with the trip signal. In another embodiment, the triperror signal causes the alert module 606 to open the contact 108. Inanother embodiment, the trip error signal causes the microprocessor 808,the watchdog apparatus 134, or other device to reset. One of skill inthe art will recognize other ways to utilize the trip error signal.

In one embodiment, the watchdog apparatus 134 includes at least aportion of a microprocessor 808 in the ground monitor apparatus 102. Forexample, the watchdog signal module 602 with the first pulse module 702and the second pulse module 708 may be portions considered part of thewatchdog apparatus 134 in the microprocessor 808. By having themicroprocessor 808 generate the first pulse signal and/or the secondpulse signal, the watchdog apparatus 134 may provide a way to monitorhealth of the microprocessor 808. For example, if one or both of thefirst pulse signal and the second pulse signal stop functioning, changepulse width, etc., this may indicate that the microprocessor 808 is notfunctioning. By having the signal monitoring module 604 possibly with afirst pulse average module 704 and a first pulse window module 706, thesecond pulse average module 710, the second pulse window module 712, andthe alert module 606 external to the microprocessor 808, these externalmodules 604, 704, 706, 710, 712, 606 may continue to function in theevent of a failure of the microprocessor 808 and can send an alertsignal.

In one embodiment, the watchdog apparatus 134 includes a power supplyfailure module 810 that determines failure of one or more power suppliesproviding power to the watchdog apparatus 134. The alert module 606sends the alert signal in response to the power supply failure module810 determining that the analog power supply and/or the digital powersupply has failed. Typically the ground monitor apparatus 102, thewatchdog apparatus 134, and/or the power source 104 includes one or morepower supplies that provide power to circuits, to the microprocessor808, and to other equipment known in the art. The one or more powersupplies may be analog, digital, or other type of power supply thatprovides power. Typically when a power supply fails, equipment receivingpower from the power supply stops functioning or malfunctions. The powersupply failure module 810 monitors the power supplies for failure andsends the alert signal which opens the contact 108 when the power supplyfailure module 810 determines that a failure condition exists in a powersupply.

FIG. 9 is a schematic block diagram illustrating one embodiment of amethod 900 for a watchdog function in accordance with an embodiment ofthe present invention. The method 900 begins and generates 902 awatchdog signal. The watchdog signal is generated at a specific intervaland includes an identifiable characteristic. The watchdog signal is apart of a device being monitored for functionality, such as amicroprocessor 808 or other device. The method 900 determines 904 if thewatchdog signal includes the identifiable characteristic. If the method900 determines 904 that the watchdog signal includes the identifiablecharacteristic, the method 900 returns and generates 902 the watchdogsignal. If the method 900 determines 904 that the watchdog signal doesnot include the identifiable characteristic, the method 900 sends 906 analert signal, and the method 900 ends.

FIG. 10 is a schematic block diagram illustrating another embodiment ofa method 1000 for a watchdog function in accordance with an embodimentof the present invention. The method 1000 begins and generates 1002 afirst pulse signal of a first frequency. The first pulse signal includesa pulse of a first pulse width. The method 1000 determines 1004 a firstpulse average. The first pulse average includes an average level of oneor more periods of the first pulse signal. The method 1000 determines1006 if the first pulse average is between a first pulse maximumthreshold and a first pulse minimum threshold. If the method 1000determines 1006 that the first pulse average is between a first pulsemaximum threshold and a first pulse minimum threshold, the method 1000returns and generates 1002 the first pulse signal. If method 1000determines 1006 that the first pulse average is not between a firstpulse maximum threshold and a first pulse minimum threshold, the method1000 sends 1008 an alert signal, and the method 1000 ends.

In addition, the method 1000 generates 1010 a second pulse signal of asecond frequency. The second pulse signal includes a pulse of a secondpulse width. The first pulse signal and the second pulse signal may bedifferent or the same. The method 1000 determines 1012 a second pulseaverage where the second pulse average includes an average level of oneor more periods of the second pulse signal. The method 1000 determines1014 if the second pulse average is between a second pulse maximumthreshold and a second pulse minimum threshold. If the method 1000determines 1014 that the second pulse average is between a second pulsemaximum threshold and a second pulse minimum threshold, the method 1000returns and generates 1010 the second pulse signal. If method 1000determines 1014 that the second pulse average is not between a secondpulse maximum threshold and a second pulse minimum threshold, the method1000 sends 1008 an alert signal, and the method 1000 ends.

FIG. 11 is a schematic block diagram illustrating a third embodiment ofa method 1100 for a watchdog function in accordance with an embodimentof the present invention. The method 1100 begins and generates 1102 afirst pulse signal of a first frequency. The first pulse signal includesa pulse of a first pulse width. The method 1100 determines 1104 a firstpulse average. The first pulse average includes an average level of oneor more periods of the first pulse signal. The method 1100 determines1106 if the first pulse average is between a first pulse maximumthreshold and a first pulse minimum threshold. If the method 1100determines 1106 that the first pulse average is between a first pulsemaximum threshold and a first pulse minimum threshold, the method 1100returns to the beginning. If method 1100 determines 1106 that the firstpulse average is not between a first pulse maximum threshold and a firstpulse minimum threshold, the method 1100 sends 1108 an alert signal, andthe method 1100 ends.

In addition, the method 1100 generates 1110 a second pulse signal of asecond frequency. The second pulse signal includes a pulse of a secondpulse width. The first pulse signal and the second pulse signal may bedifferent or the same. The method 1100 determines 1112 a second pulseaverage where the second pulse average includes an average level of oneor more periods of the second pulse signal. The method 1100 determines1114 if the second pulse average is between a second pulse maximumthreshold and a second pulse minimum threshold. If the method 1100determines 1114 that the second pulse average is between a second pulsemaximum threshold and a second pulse minimum threshold, the method 1100returns to the beginning. If method 1100 determines 1114 that the secondpulse average is not between a second pulse maximum threshold and asecond pulse minimum threshold, the method 1100 sends 1108 an alertsignal, and the method 1100 ends.

The method 1100 also determines 1116 if a fault condition exists in thepower source 104, the ground monitor apparatus 102, etc. If the method1100 determines 1116 that a fault condition does not exist, the method1100 returns to the beginning. If the method 1100 determines 1116 that afault condition exists, the method 1100 generates 1118 a trip signal andthe method 1100 sends 1108 an alert signal. The method 1100 alsodetermines 1120 if the trip signals, one being active high and anotherbeing active low, are the same. If the method 1100 determines 1120 thatthe trip signals are not the same, the method 1100 returns to thebeginning. If the method 1100 determines 1120 that the trip signals arethe same, the method 1100 sends 1108 an alert signal. The method 1100determines 1122 if a power supply has failed. If the method 1100determines 1122 that a power supply has not failed, the method 1100returns to the beginning and generates 1102, 1104 the first and secondpulse signals, determines 1116 if a fault condition exists, determines1120 if the trip signals are the same, and determines 1122 if a powersupply has failed. If the method 1100 determines 1122 that a powersupply has failed, the method 1100 sends 1108 an alert signal, and themethod 1100 ends.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. An apparatus comprising: a signal generation module that injects a signal in a pilot conductor, the signal comprising a direct current (“DC”) component and one or more alternating current (“AC”) components, each AC component of the one or more AC components comprising a frequency different from other AC components of the one or more AC components, wherein current in the pilot conductor is injected in a grounding conductor of a set of power cables connecting a power source to a load, the grounding conductor connected to a ground return, wherein the set of power cables, the grounding conductor and the pilot conductor are combined from the power source to the load; a current monitor module that monitors current in at least one of the pilot conductor and the ground return; a DC detection module that determines a DC current present in the current monitored by the current monitor module; an AC detection module that determines an AC current corresponding to each frequency of the one or more AC components present in the current monitored by the current monitor module; a DC minimum threshold module that determines if the DC current is below a DC current minimum threshold; an AC threshold module that determines if one or more of the AC currents determined by the AC detection module is below an AC threshold; and a trip module that opens a contact in response to one or more of the DC threshold module determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and the AC threshold module determining that at least one of the AC currents is below an AC threshold for longer than an AC current time threshold, the contact disconnecting the power source from the set of power cables.
 2. The apparatus of claim 1, further comprising a DC maximum threshold module that determines if the DC current is above a DC current maximum threshold and wherein the trip module further opens the contact in response to the DC maximum threshold module determining that the DC current is above the DC maximum current threshold.
 3. The apparatus of claim 2, wherein the DC current maximum threshold corresponds to a current that is above a DC current in the pilot conductor or ground return that is at a level indicative of an operating condition without a stray DC current component.
 4. The apparatus of claim 2, further comprising a pilot wire diode (“PWD”) that comprises one or more diodes, wherein the PWD is connected to the pilot conductor and to the grounding conductor and wherein he PWD has a voltage drop when the signal is applied to the pilot conductor.
 5. The apparatus of claim 4, wherein the DC current maximum threshold corresponds to a current that is above a DC current in the pilot conductor or ground return when the load comprises a properly functioning PWD.
 6. The apparatus of claim 4, wherein the PWD is located at the load.
 7. The apparatus of claim 1, wherein the trip module opens the contact in response to the AC threshold module determining that for at least two of the AC currents, an AC current is below an AC threshold for longer than an AC current time threshold.
 8. The apparatus of claim 7, wherein the AC current time threshold comprises a first AC current time threshold and wherein the one or more AC components comprise three AC components and wherein the trip module further opens the contact in response to the AC threshold module determining that each of the three AC currents is below an AC threshold for longer than a second AC current time threshold.
 9. The apparatus of claim 1, further comprising a filter module that separates each of the DC component and the one or more AC components from each other.
 10. The apparatus of claim 9, wherein the filter module further substantially removes a power source fundamental AC component and harmonics of the power source fundamental AC component from within the one or more AC components of the current monitored by the current monitor module.
 11. The apparatus of claim 1, wherein the one or more AC components comprise three AC components and wherein a frequency of each AC component of the three AC components comprises a frequency that is not a harmonic of a frequency of the other AC components and is not a harmonic of a fundamental frequency of the power source.
 12. The apparatus of claim 11, wherein the frequencies of the three AC components comprise 85 hertz (“Hz”), 208 Hz, and 990 kilohertz (“kHz”).
 13. The apparatus of claim 1, further comprising: a return current module that determines a current in the grounding conductor; and a return current threshold module that determines if the current in the grounding conductor is below a return current threshold, wherein the trip module further opens the contact in response to the return current threshold module determining that the current in the grounding conductor is below the return current threshold for a period longer than a return current time threshold.
 14. The apparatus of claim 1, further comprising: a ground fault current module that determines a current in a connection between a ground of the power source and a chassis ground of the power source; and a ground fault threshold module that determines if the current determined by the ground fault module is above a ground fault threshold, wherein the trip module further opens the contact in response to the ground fault threshold module determining that the current determined by the ground fault current module is above the ground fault threshold for a period longer than a ground fault time threshold.
 15. The apparatus of claim 1, wherein the DC current minimum threshold corresponds to a combined pilot conductor and grounding conductor resistance of less than 50 ohms.
 16. The apparatus of claim 1, wherein the DC detection module isolates a DC component of the current monitored by the current monitor module and the AC detection module determines the AC current corresponding to each frequency of the one or more AC components present in the current monitored by the current monitor module by isolating each AC current with a frequency of the one or more AC components.
 17. A system comprising: a power source; a ground monitor in the power source, the ground monitor comprising a signal generation module that injects a signal in a pilot conductor, the signal comprising a direct current (“DC”) component and one or more alternating current (“AC”) components, each AC component of the one or more AC components comprising a frequency different from other AC components of the one or more AC components, wherein current in the pilot conductor is injected in a grounding conductor of a set of power cables connecting the power source to a load, the grounding conductor connected to a ground return, wherein the set of power cables, the grounding conductor and the pilot conductor are combined from the power source to the load; a current monitor module that monitors current in at least one of the pilot conductor and the ground return; a DC detection module that determines a DC current present in the current monitored by the current monitor module; an AC detection module that determines an AC current corresponding to each frequency of the one or more AC components present in the current monitored by the current monitor module; a DC minimum threshold module that determines if the DC current is below a DC current minimum threshold; an AC threshold module that determines if one or more of the AC currents determined by the AC detection module is below an AC threshold; and a trip module that opens a contact in response to one or more of the DC threshold module determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and the AC threshold module determining that at least one of the AC currents is below an AC threshold for longer than an AC current time threshold, the contact disconnecting the power source from the set of power cables.
 18. A method comprising: injecting a signal in a pilot conductor, the signal comprising a direct current (“DC”) component and one or more alternating current (“AC”) components, each AC component of the one or more AC components comprising a frequency different from other AC components of the one or more AC components, wherein current in the pilot conductor is injected in a grounding conductor of a set of power cables connecting a power source to a load, the grounding conductor connected to a ground return, wherein the set of power cables, the grounding conductor and the pilot conductor are combined from the power source to the load; monitoring current in at least one of the pilot conductor and the ground return; determining a DC current present in the current monitored in one or more of the pilot conductor and the ground return; determining an AC current corresponding to each frequency of the one or more AC components present in the current monitored in one or more of the pilot conductor and the ground return; determining if the DC current is below a DC current minimum threshold; determining if one or more of the AC currents corresponding to a frequency of the one or more AC components in the monitored current is below an AC threshold; and opening a contact in response to one or more of determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and determining that at least one of the AC currents is below an AC threshold for longer than an AC current time threshold, the contact disconnecting the power source from the set of power cables.
 19. The method of claim 18, further comprising determining if the DC current is above a DC current maximum threshold and further comprising opening the contact in response to determining that the DC current is above the DC maximum current threshold.
 20. The method of claim 19, wherein the DC current maximum threshold corresponds to a current that is above a DC current in the pilot conductor or ground return that is at a level indicative of an operating condition without a stray DC current component. 